The invention relates specifically to a transmission apparatus for transmission of electrical signals between a line driver, which is connected to a transmission line, and a switching center, having:
a) an analog feedback device for coarse setting of a line voltage level on the transmission line as a function of a line current which is detected in the line driver; and
b) a digital feedback device, which has a feedback filter device for fine setting of an amplitude and phase relationship between the line voltage level and the line current.
FIG. 3 shows a coding/decoding unit (Codec) based on the prior art. The coding/decoding unit is connected via a filter F2 to a line driver (SLIC=Subscriber Line Interface Circuit). The line driver is connected via a further filter F1 to a transmission line L, which is represented in FIG. 3 by a (generally complex) resistor. The line driver contains a power amplifier and a current sensor device for detection of the current which is flowing in the line. The coding/decoding unit Codec is required, inter alia, in order to set a country-specific input impedance.
Normally, this is done by means of programmable analog and digital feedback loops AS, DS1 and DS2. The coding/decoding unit (CODEC) illustrated in FIG. 3 comprises an analog circuit part and a digital circuit part, which are isolated from one another by an analog/digital converter ADC and/or a digital/analog converter DAC. As illustrated, the analog part contains an analog feedback loop AS, while the digital part contains two digital feedback loops DS1 and DS2.
The signal which is emitted from the current sensor is supplied via the filter F2 to a pre-filter PREFI for the coding/decoding unit. The signal which is emitted from the pre-filter PREFI is supplied on the one hand to the analog/digital converter ADC and on the other hand is fed back in an analog feedback loop AS, and is supplied to a summation device “+”. The analog feedback loop AS makes it possible to set a line voltage level on the transmission line as a function of a line current which is detected in the line driver.
The digital feedback loops DS1 and DS2 which are provided in the digital part of the coding/decoding unit are used for fine setting of an exact amplitude and phase relationship between the line voltage level and the line current. The feedback loop DS1 feeds a signal, which is output between two decimation units D1 and D2, back to a summation unit which is arranged between two interpolation units I1 and I2.
The feedback loop DS2 feeds the output signal from the decimation unit D2 back to the input of the interpolation unit I1 by superimposing the signal to be transmitted on the feedback signal in a summation unit “+”. The signal which is emitted from the second interpolation unit I2 is converted in the digital/analog converter DAC to an analog signal and is supplied to a post-filter unit POFI. The signal which is emitted from the post-filter unit POFI has the analog feedback signal that is produced by the analog feedback loop superimposed on it. This means that the conventional circuit arrangement illustrated in FIG. 3 can be used to provide both a line voltage level and a phase relationship between the voltage and the current, that is to say complex line impedance matching.
The conventional circuit arrangement illustrated in FIG. 3 for setting a complex line impedance has significant disadvantages, however. The first feedback loop DS1 contained in the digital part has to operate at a high sampling rate, since the feedback signal is in fact extracted after the decimation by the first decimation device D1. This means that this digital feedback loop has to be designed for a high signal and sampling rate. A further disadvantage is that the digital feedback loop DS1 comprises a recursive filter unit, which is less flexible than a non-recursive filter unit.
A further disadvantage is that it is difficult to reduce the number of coefficients in a recursive filter unit which is arranged as shown in FIG. 3. A recursive filter unit is disadvantageously less flexible than a non-recursive filter unit.
One major disadvantage of the conventional circuit arrangement illustrated in FIG. 3 is that two digital feedback loops must be provided in the digital circuit part, in addition to the analog feedback loop in the analog circuit part. This considerably increases the circuit complexity, and disadvantageously leads to an increase in cost.